Generally speaking, in order to preserve the quality of the contents in hermetically sealed containers such as canned provisions, such containers have therein gaps and have negative or positive pressures. Accordingly, the air-tightness in sealed containers such as beverage cans may be effectively determined in a nondestructive manner by inspecting their internal pressures.
There is conventionally known a manual hammering inspection method, according to which the top surface of a sealed container is gently striken manually with a rod-shape hammer and thus generated vibration is checked based on the sound, reaction and eye measurement, so that airtightness is judged. However, it requires much skill and patience to preform quickly and accurately the inspection of a large number of sealed containers according to such a manual hammering inspection method.
On the other hand, in a sealed container having a negative pressure, there is a case where the inflow of the outside air through small scratches or holes formed at the time the container is charged with contents or hermetically sealed, is slowly advanced with the passage of one or two weeks, and provokes a slow pressure leak phenomenon, resulting in an excessively decreased pressure of a zero pressure in the container. There is also a case where, by the change in quality of the contents in a container with the passage of time, the internal pressure which should normally be a negative pressure becomes a positive pressure and subsequently such container is expanded. However, it is very difficult to find such a slow pressure leak or can expansion phenomenon immediately after the container has been charged with contents and hermetically sealed.
A conventional manual hammering inspection after the passage of a predetermined period of time is performed in such a manner that containers charged and hermetically sealed are first housed in cases and such cases are stored and preserved for a predetermined period of time, after which the containers taken out from the cases are subjected to the hammering inspection. Thereafter, the containers are again housed in the cases and the cases are again closed, thus requiring much labor with the possibility of the breakage of cases and/or the damages of containers accompanied. Thus, such conventional manual hammering inspection is very inconvenient.
There is also known a mechanical automatic inspection method, according to which a pulsated electromagnetic force is generally exerted to the lid of each can to generate vibration on the can lid. From the fact that the relationship between the fundamental vibration frequency of vibration sound and the pressure of the article to be inspected becomes as shown in FIG. 1, the natural damped vibration of the lid is detected acoustically or electromagnetically, and a signal obtained from such detection is passed through filters. Then, it is checked whether or not a predetermined frequency band width of the electric signal passed through the filters is within a predetermined frequency range, and also the level of a signal generated by the fundamental vibration is checked, whereby the can container is judged as good or defective.
However, there is a case where the acoustic judgment is affected by the external noise conditions, so that the inspection quality is decreased.
Furthermore, can containers judged as defective include containers having insufficient negative pressures or atmospheric pressures in which the originally normal internal pressures have leaked due to the insufficiency of the charging temperature or the imperfect integration of can lids with can main bodies, and also include deformed or expanded containers of which internal pressures have become positive pressures for some reasons.
The wave-form characteristics of electric signals actually obtained from good and defective can containers are shown in FIGS. 2 and 3, in which the frequency is taken on the abscissa in a uniform scale and the signal level is taken on the ordinate in a logarithmic scale.
FIG. 2 (a) illustrates the wave-form obtained from a good can container of which internal pressure is a predetermined negative pressure. FIG. 2 (b) illustrates the waveform obtained from a can container of which internal pressure is decreased in the vicinity of the standard lower limit. FIG. 2 (c) illustrates the wave-form obtained from a can container having no internal pressure.
FIG. 3 (a) to (c) illustrate the wave-forms obtained from expanded can containers which have been insufficiently sealed due to defective integration of can lids with can bodies and their internal pressures become positive pressures. FIG. 3(a ) illustrates the wave-form obtained from a can container of which internal pressure is relatively low. FIG. 3(b) illustrates the wave-form obtained from a can container of which the absolute value of internal pressure is equivalent to the internal pressure of a good can container. FIG. 3(c) illustrates the wave-form obtained from an expanded can of which internal pressure is further increased in the vicinity of the can breaking pressure.
In FIGS. 2 and 3, the highest levels and the subsequent levels of signals generated by the fundamental vibration are continuously plotted and the corresponding frequencies are designated as f1, f2, f3 and f4, respectively, from the lowest frequency. Then, multipliers .alpha. of 2 to 3 are obtained from the relationship of the frequencies f4 with respect to the frequencies f1, respectively. Namely, where the signals at the frequencies f1 are designated as fundamental frequency signals and the signals at the frequencies around the "f1.times..alpha." are designated as higher harmonic signals, it is theoretically possible to judge from the relationship between the fundamental frequency signals and the higher harmonic signals, that a certain can container is one shown in FIG. 2 or an expanded can shown in FIG. 3. The appropriateness of such judgment is proved from other similar data.
However, according to a conventional inspection method, distinction between the fundamental frequency signal and the higher harmonic signal has not been made, and therefore there has been a case where an expanded can cannot be distinguished from a good can having a normal negative pressure. Thus, such conventional inspection method lacks the reliability.
When the second largest level signal subsequent to the fundamental frequency signal is present at the frequency f2, the level of the fundamental frequency signal is affected by the inspection timing, so that the inspection result may become inaccurate. It is therefore necessary to allow a sufficient tolerance for the limit of good can container, thus further decreasing the reliability of the inspection.
It is also found from a large number of data that the signal level at the lowest frequency f1 is not always higher than the signal level at the second lowest frequency f2. This means such inspection method includes an inaccurate factor and therefore highly accurate inspection cannot be expected.
When a great number of sealed containers such as cans are arranged in a case for example in a corrugated cardboard box and the internal pressures of such sealed containers as housed in the case are externally inspected, it is necessary to accurately detect and inspect the center portions of the lids of such sealed containers. However, the sealed containers are not always arranged in accurate position in line in the box, but in most cases it seems they are positionally shifted. When such positionally shifted containers are vibrated with a pulsated electromagnetic force exerted to the portions thereof apart from the centers, their detected wave-forms become different from those detected along the vertical lines extending from the accurate centers of the sealed containers. In consequence, such inspection may produce an erroneous judgment, thus causing the inspection accuracy to be lowered.
With respect to such positional shifting, wave-forms obtained at the time of inspection of a non-gaseous drink can, are shown by way of example.
FIG. 4 illustrates the wave-forms obtained from a can of which internal pressure is a normal negative pressure. FIG. 4(a) illustrates the wave-form detected at a position on the portion 10 mm apart from the can lid center, FIG. 4(b) illustrates the wave-form detected at a position on the portion 5 mm apart from the can lid center, and FIG. 4(c) illustrates the waveform detected at a position on the can lid center.
FIG. 5 illustrates the wave-forms obtained from a defective can of which internal pressure is zero or equal to the atmospheric pressure. FIGS. 5(a), (b) and (c) illustrate the wave-forms detected at positions 10 mm, 5 mm and 0 mm apart from the lid center of the same defective can, respectively.
In comparison of FIG. 4 with FIG. 5, the frequencies f1 exhibiting the fundamental frequency signals having the highest level are constant regardless of the positional shifting. Therefore, such wave-forms may be used as inspection data of the can internal pressures. However, the level of signals other than the fundamental frequency signal, for example the level of the signal at the frequency f2, varies with the respective distances between the inspected positions and the lid centers.
FIG. 6 illustrates the wave-forms detected from an expanded can of which internal pressure which should normally be a negative pressure, becomes a positive pressure for some reason. The absolute value of the internal pressure in the can shown in FIG. 6 is the same as that of the can shown in FIG. 4, and in FIG. 6 the fundamental frequency signals are located at the similar frequencies in the case of FIG. 4. As the consequence, there is a possibility of the internal pressure of the can in FIG. 6 being judged as normal. Therefore, the can in FIG. 6 may be regarded as good one, although it should absolutely be detected as defective one.
As shown in FIG. 6, the expanded can has a characteristic that the higher harmonic signal level at the frequency f4 becomes extraordinary high when inspection is performed on the center of the can lid. However, as shown in FIG. 6(a) and (b), when inspection is performed on the portions apart from the center of the can lid, the higher harmonic signal levels vary with the respective distances between the inspected positions and the can lid center in substantially inverse proportion, and there is a case where the higher harmonic signal disappears. Accordingly, judgment of whether a certain can has an excessive pressure or has a normal negative pressure may become inaccurate dependent on the positional shift of the can potentially taken place in the can storing case.
Accordingly, it is apparently necessary to perform the inspection of the can as not positionally shifted in the case. However, it is not easy to detect accurately the center positions of the lids of a large number of cans arranged in the case. Practically, there is applied an indirect inspection method, according to which the bodies of the cans arranged at the both ends of the case are magnetically detected and the positions of the cans arranged at the inner part of the case are supposed, or the outer surfaces of the case are detected and the can positions in the case are calculated. However, in any case, no accurate position detection may be performed.